Transmissive optical encoders are widely used to detect rotational movement. They consist of a "code wheel" attached to the rotating member whose movement needs to be monitored. The code wheel passes in between a light emitter and detector. On the code wheel are different "tracks"; patterns to block light or let light pass through. The interruptions of the light beam from the emitter are detected by the detector and signals are generated to determine rotational movement. Some manufacturers, including Hewlett-Packard, offer a "module" that contains an emitter and a detector in a package.
An absolute encoder indicates the absolute position of a rotating shaft upon power-up without any movement. An incremental 3-channel encoder with two quadrature signals and high resolution index signal may be used to detect absolute position within some movement if a unique pattern of index pulses is used. Since some movement is needed before absolute position can be determined after power-up, the encoder is called a "quasi-absolute" encoder. After finding absolute position, the encoder can keep track of incremental movement with high precision.
A 3-channel optical encoder, e.g. Hewlett-Packard HEDS 9140 that includes a 3-channel emitter and detector module, and a 3-channel code wheel generates channel A (CH A), channel B (CH B), and channel I (CH I). Signals CH A and CH B are in quadrature and are offset from each other by 90.degree. in phase. These two signals are used to detect incremental change in position and direction of movement. Channel CH I is typically used as a signal that occurs once for every rotation of the code wheel and it indicates an "absolute zero" (or reference) position. A high resolution index track allows index pulses to occur on every cycle of signal CH A or CH B. A unique sequence of index pulses is used to determine the absolute position of a rotating member.
Many applications, e.g. brushless motor commutation and short local homing sequences for robotics, require absolute position sensing. A quasi-absolute solution is desirable because an incremental 3-channel encoder is less expensive than an absolute encoder and the incremental signals can be used for high precision position tracking once the absolute position is found.